Crude oil can be directly injected into a gas chromatograph to determine compounds like methanol. A less abrasive injection technique is to sample the headspace.
Methanol is a highly volatile compound that partitions easily into the headspace from crude oil. It is used in crude oil to prevent gas hydrates forming in pipelines. However, it can lead to costly problems if residual methanol in crude oil negatively impacts water treatment facilities.
Direct injection of crude oil into a gas chromatograph, as done with traditional methods such as ASTM D7059, requires backflush capability to properly analyze methanol with a flame ionization detector.
Using SH-GC-VUV to Determine Methanol Content
Static headspace – gas chromatography – vacuum ultraviolet spectroscopy (SH-GC-VUV) can be used as an alternative method in order to avoid direct injection of crude oil. The GC configuration is simplified with this technique and the GC inlet and column to from crude oil contamination. Spectral identification and quantification of methanol are also provided.
An example chromatogram of methanol in an ASTM light crude oil, using SH-GC-VUV is shown in Figure 1. The methanol is well separated from water and other compounds as depicted with the peak, which is baseline-resolved.
Comparable to existing methods, the inset plot shows that methanol can be detected down to 5 ppm. Good linearity was shown in SH-GC-VUV calibration curves for methanol in crude oil from 5 to 1000 ppm (Figure 2).
Figure 1. Chromatogram of crude oil spiked with various levels of methanol, and 1-propanol and 2-propanol as internal standards, all baselined resolved. All crude oil compounds elute in under 20 minutes.
Figure 2. Duplicate calibration curve, covering 5ppm to 1000 ppm. The curve is linear with an R2 greater than 0.99.
Headspace GC-VUV: A Viable Alternative
Figure 3 emphasizes the importance of VUV absorbance spectra with an example of methanol, 2-propanol, and 1-propanol. They are all classified as alcohols yet each VUV spectrum is unique, even for the two isomers.
Figure 3. VUV absorbance spectra of methanol, 2-propanol and 1-propanol.
This demonstrates that headspace GC-VUV is a viable alternative to analyze volatiles in crude oil.
This information has been sourced, reviewed and adapted from materials provided by VUV Analytics.
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